Regular Expression Mastery

Backreference Numbering is Lexical

        # $file is "report.pl" or "/usr/local/bin/report.pl"
        ($path, $name, $suff) = $file =~ m{ (.*/)?(.*)\.(.*)};

        /usr/local/bin/  report  .   pl
        -----path------  -name-     suff
              $1           $2        $3

But what about when $file is report.pl and has no path?
Since the (.*/)? is 'skipped', will (.*)\.(.*) be $1 and $2?
No. The parentheses are numbered at compile time
- The value of $file cannot affect that

                         report  .   pl
             path        -name-     suff
              $1           $2        $3

$path here is undefined
Similarly /(a+)|(b+)/
- If there are any a's, they will be in $1
- If there are b's but no a's, the b's will be in $2, and $1 will be undefined
- $1 always contains the a's; $2 always contains the b's